The invention relates to a method of detecting a toner concentration in a developer which is used in a magnetic developing system.
In a magnetic developing system, an electrostatic latent image is converted into a visual image by a developer supplied thereto through magnetic means and which comprises mixture of a carrier particles of a magnetic material and a toner of a non-magnetic material. Since only the toner particles migrate to the latent image under the electrostatic interaction as the developer is supplied thereto, the toner content in the developer is gradually reduced when the developing process is repeated. However, the ratio or proportion between the toner and the carrier contained in the developer represents a controlling factor on the developing performance. If the toner component within the developer is too low as compared with the carrier component, the resulting optical density of the visual image will be insufficient, and a visual image with a low contrast will result. Conversely, if the toner content is excessively high, the toner will attach to a non-magnetic area during the developing process, producing a so-called background smearing. Therefore, it is essential, in order to achieve a proper magnetic developing process in a satisfactory manner, to maintain the ratio or proportion of the carrier and the toner contained in the developer in a proper range, by replenishing with an additional amount of toner. A proper range for the proportions of the carrier and the toner is considered to be from 3 to 5 percent by weight of the toner in the overall developer when the developer comprises a mixture of iron powder as the carrier with the toner.
It is necessary to detect the proportion of the toner relative to the carrier, or the toner concentration in the developer, in order to properly replenish the toner. Since the carrier is magnetizable while the toner is not, a change in the relative proportion of the carrier and the toner contained in the developer results in a change in the magnetic permeability thereof. Therefore, there has been proposed a method of detecting a toner concentration in a developer which comprises the steps of forming the developer into a given configuration, placing it at a given position within a magnetic field formed by a fixedly mounted magnet to thereby define a magnetic path in the developer, determining a leakage flux from the developer at another given position, and detecting toner concentration in accordance with a predetermined relationship between a change in the leakage flux and a change in the toner concentration. Under practical conditions, the above mentioned change in the leakage flux is small, on the order of several tens of Gauss, which, however, can be detected with sufficient accuracy by employing a Hall element, in particular a Hall element comprising evaporated indium antimony.
However, while the Hall element exhibits a very high sensitivity, its output is strongly dependent on the temperature, so that the magnitude of the output varies as the temperature varies, even though the strength of the magnetic field as the input remains constant, thus requiring a special temperature compensation or a thermostatic oven.